1. Field of the Invention
The present invention relates to devices for scanning and, more specifically, to devices for optical scanning along a substantially linear path.
2. Description of the Prior Art
Devices for scanning beams of light have long been known to the art. Optical scanning is used in a variety of applications, including writing and reading data to and from such storage media as compact discs and optical data cards. Optical data cards, and other types of linear-track optical data storage media, store data along linear tracks. Scanning such tracks requires linear translation of either the data card or the device used to scan the data card. Current devices include mechanical systems, electronic systems, acousto-optical systems, electro-optical systems, and other systems for moving an optical beam along a path. Most mechanical devices employ rotating polygon mirrors or prisms, galvanometer actuators carrying mirrors, and similar devices.
In the case of linear-track optical data storage media, current scanning devices incorporate Optical Pickup Units (OPU) which reciprocate relative to a data storage medium from the beginning to the end of the data track. The OPUs are typically mounted on a carriage assembly which is constrained to move in a straight line parallel to a data track. A linear actuator imparts a force on the carriage assembly to effect the linear motion and linear position transducer determines instantaneous OPU location and provides feedback for the velocity control function on of the actuator. During a typical data scan, the OPU translates down the track, illuminating the data spots on the track with an optical beam and receives reflected signals by means of electro-optical components. Current devices require acceleration of the OPU to its operating speed at the beginning of each data track scan and deceleration of the OPU to a stop at the end of each data track. This motion is then repeated in each direction.
Such back and forth motion of the OPU and carriage assembly results in undesirable acceleration and deceleration of the OPU and vibration and ultimately limits the operating speed of the device. Scan speed may be increased by decreasing the mass and friction associated with the OPU and carriage assembly or by increasing the force provided by the actuator.
In addition to translating back and forth down the data tracks to read and write optical data, the OPU must provide small amplitude, high speed focus and cross-track motions. This is because the data spot size is on the order of a single micrometer diameter and the track-to-track spacing is typically on the order of ten micrometers. As the OPU scans along the data track, small imperfections in any realizable mechanical mechanism result in tiny motions perpendicular to the data track, and therefore failure to maintain the required alignment between the optical data and the OPU. To compensate for misalignment caused by these undesirable motion, current design practices incorporate high speed actuators as part of the objective lens mounting assembly to deflect the beam in the cross track direction (perpendicular to the data track in the plan of the optical medium) and also in the "focus" direction (perpendicular to the data track, normal to the optical medium). Since the actuators only move the objective lens (whose mass can be made relatively small), high speed compensation of small tracking and focus errors may be realized. Tracking and focus error signals which drive the compensator actuators are usually developed by auxiliary optical and electronic components within the OPU.
Other mechanical means for optical scanning, such as spinning polygon mirrors or galvanometer driven mirrors, are common in other applications, but are not used in scanning optical storage media for two reasons. First, they scan a focused beam onto a curved (cylindrical) surface. If the curvature is compensated by optical elements, neither the data track illumination nor subsequent reflection is normal to the planar surface of the storage medium. Optical data storage media ordinarily require the illuminating beam to be focused to a small spot and require it to strike the surface of the medium at substantially perpendicular incidence. The resulting reflection also propagates perpendicularly bact from the surface through the same optical train as the illuminating beam. One method attempts to circumvent this problem by deforming the card to conform to a cylindrical surface. The focused illumination spot follows a circle which is supposed to be coincident with a data track on the surface of the deformed card. However, the card must be bent in such a way that its surface is accurately coincident with the required cylindrical surface to within a few micrometers (otherwise the fast focus compensation mechanism will be unable to maintain acceptable focus as the spot moves along the data track). Such an approach has two disadvantages. First, considering the relatively simple and inexpensive procedures and materials used in manufacturing optical memory cards (OMCs), it is hard to achieve necessary accuracy when the card is bent. Second, deforming the card may result in excessive wear on the card and may also introduce birefringence in the transparent protective covering of the data card with undesirable effects on the polarization state of the illumination and reflected beams.
An advantage of the present invention is that it does not require reciprocating components. Thus, it reduces drive power, reduces vibration and it offers the potential for increased speed.
A further advantage of the invention is that it maintains the scan beam direction perpendicular to the surface of the object being scanned.
A further advantage of the invention is that the OPU remains essentially stationary, thereby reducing vibration, drive power and design complexity.